Diesel engines, some gasoline fueled engines and many hydrocarbon-fueled power plants are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. The hot exhaust gas produced by such lean-burn engines generally contains a relatively high concentration of oxygen (about one to ten percent by volume) and water, as well as unwanted gaseous emissions that may need to be converted to more innocuous substances before being discharged to the atmosphere. The gaseous emissions primarily targeted for abatement include carbon monoxide (CO), unburned and partially burned hydrocarbons (HC), and nitrogen oxide compounds (NOX). The NOX constituent in the exhaust gas produced by a lean-burn engine comprises mostly NO (greater than 90 mol %) with some NO2 (less than 10 mol %) and nominal amounts of N2O. To the extent that the hydrocarbon fuel contains sulfur, the exhaust gas may also contain sulfur dioxide (SO2).
Exhaust gas treatment systems that include specially catalyzed flow-through reactors are commonly used to effectively treat exhaust gas flows. In general, these treatment systems—and the catalyst materials therein—are designed to promote (1) the oxidation of CO to CO2, (2) the oxidation of HC to CO2 and water, and (3) the reduction of NOX to N2 and water.
In general, the HC, CO, NOX and oxygen (O2) content of an exhaust flow from an internal combustion engine is not constant, but changes due to variations in the air-to-fuel mass ratio (AFR) of the combustible mixture delivered to the engine. The stoichiometric AFR of a combustible mixture of air and fuel is when exactly enough oxygen in the supplied air is present to completely burn all of the fuel. For standard gasoline fuel, the stoichiometric AFR is 14.7:1. Stoichiometric combustible mixtures and mixtures that contain an excess of fuel (i.e., AFR≦14.7) are referred to as “rich.” Engines that operate by burning rich mixtures of air and fuel produce an exhaust flow with relatively high amounts of unburned or partially burned fuel (e.g., CO and HC) and small amounts of O2. On the other hand, combustible mixtures that contain an excess of air (i.e., AFR>14.7:1) are referred to as “lean.” Exhaust flows from engines that operate by burning lean combustible mixtures contain relatively high amounts of O2 and NOX (about 90 mol % NO, less than 10 mol % NO2 and nominal amounts of N2O) and relatively low amounts of CO and HC.
Variations in the AFR of a combustible mixture may be intentional, for example, in order to optimize the fuel-efficiency of the engine. Alternatively, the AFR of combustible mixtures may oscillate above and below a predetermined target value due to frequent accelerations and decelerations of the vehicle. In either case, effective exhaust aftertreatment systems for automotive vehicles must be tolerant to such changing exhaust flow conditions so that the variable amounts of HC, CO and NOX in the exhaust flow are continuously treated.
Several treatment systems have been developed for vehicle applications. One conventional approach is to use a selective catalytic reduction (SCR) system to promote the reduction of NOX in a high-oxygen content exhaust flow. An SCR operates by injecting a reductant material, such as ammonia (NH3) or hydrocarbons (HC), into the exhaust flow before it is passed in contact with an NOX reduction catalyst. The reductant material reacts with NOX in the presence of O2 over the NOX reduction catalyst to form N2. However, most SCR systems require a reservoir of the reductant and a dosing device to inject a controlled amount of the reductant into the exhaust flow. Additionally, the reductant must be injected far enough upstream of the reduction catalyst material to ensure uniform mixing in the exhaust gas.
An alternative catalyst system, known as a lean NOX trap (LNT), is designed to treat an NOX-containing exhaust flow from an engine that cyclically operates by burning lean and rich mixtures of air and fuel. The corresponding modes of engine operation are referred to as fuel-lean and fuel-rich, respectively. During the fuel-lean mode of engine operation, excess O2 in the exhaust flow creates an oxidizing exhaust environment, wherein NO is readily oxidized to NO2 over an NOX oxidation catalyst and is stored as a nitrate species over an NOX storage material. The engine is briefly and repeatedly operated in the fuel-rich mode to increase the amount of reductants (e.g., CO and HC) in the exhaust flow, which triggers the release of NO2 from the NOX storage material and the reduction of NO2 over a NOX reduction catalyst. Conventional LNTs use platinum (Pt) as the NOX oxidation catalyst and platinum (Pt), palladium (Pd) or rhodium (Rh) as the NOX reduction catalyst to effectively convert NOX to N2. But platinum group metals (PGMs), such as Pt, Rh and Pd, are a particularly expensive, and there is a need for less-expensive oxidation and reduction catalyst materials with equally comparable efficiency.